It is generally known that the X-ray tube and also the electric components of a computed tomograph dissipate energy in the form of heat because of their strongly fluctuating power losses.
This heat can have a negative effect on the image quality of the CT unit, since relevant components are influenced by changes in temperature during generation of images. The detector modules and the associated evaluation electronics, for example, are such temperature-sensitive components.
Furthermore, there is a risk of overheating of individual components in the CT unit. The temperature inside the gantry of a CT unit is controlled for this reason.
The DMS (=Detector Measurement System) in the gantry of the CT unit is in particular need of very accurate temperature control. As a rule, DMS includes scintillators upstream of which collimators are positioned, and photodiodes that detect the incident radiation and then relay it in the form of analogue signals. The scintillators, the collimators and the photodiodes are temperature-sensitive components. Thus, for example, temperature fluctuations can lead to longitudinal expansions and/or longitudinal contractions that are seen in undesired deformations of these components. These deformations lead, inter alia, to shading that impairs the recorded CT image.
Two separate temperature control loops have been used to date in order to be able to ensure adequate temperature management.
Firstly, a control loop controls the air temperature inside the gantry interior. In this case, use is made for the most part of ventilation elements that are driven as a function of the temperature of the gantry interior. At least one sensor determines the temperature in the interior of the gantry in this case.
An additional control loop is used for the DMS, because the temperature-sensitive DMS can be exposed to very different ambient air temperatures. Owing to the rotating gantry, the DMS can, for example, be positioned on one occasion in a thermally favorable way in front of a fan outlet, and be positioned in a thermally unfavorable fashion on another occasion.
The second control loop of the DMS mostly includes one or more controlled fans at DMS or, in the “warm-up phase of the CT unit”, heating elements inside the DMS. The temperature in the DMS that was measured using a sensor in the latter was used as controlled variable of the fans/heating elements.
This way of separately controlling the temperature of the DMS and interior of the gantry is particularly complicated.